The present invention relates to an agricultural grinder-mixer, and more particularly, to a mixing tank configuration having an improved internal mixing assembly for augmenting proper circulation of material contained in the tank.
On most present day grinder-mixers a vertical mixing assembly is mounted within a vertical mixing tank having a cylindrical upper portion and a conical downwardly converging lower portion. The mixing assembly includes a centrally located upright mixing auger rotatably mounted within a tubular housing. An inlet is provided in the housing at its lower end and a discharge opening is provided at the top of the housing. During a mixing operation, material is conveyed in a generally circuitous path, i.e., material is continually transferred from the bottom of the tank upwardly through the auger housing to the top of the tank whereupon it is discharged through the opening and allowed to gravitate back down. Commercially available apparatus of this type is illustrated by U.S. Pat. No. 3,780,993, issued Dec. 25, 1973, to Charles M. Kline, the inventor of the present invention.
The conical portion of the grinder-mixer tank described above functions to direct material inwardly towards the central mixing auger as it settles or gravitates downwardly during circulation. Since the horizontal cross section of the tank decreases substantially as the walls converge inwardly, the material is compressed as it settles causing certain crop materials, especially under moist and/or extremely tough conditions, to form a cluster of relatively solid material. This is illustrative of a problem commonly encountered during operation of tanks having a conventional conical shaped lower portion. Material moving downwardly during a mixing operation tends to build up in the area between the lower walls of the tank and the outer surface of the auger housing. This buildup, commonly referrred to as "bridging", restricts free downward flow of material and thereby impedes the circuitous flow of material and diminishes effectiveness of the central mixing auger. Certain crop conditions lead to more frequent bridging and many times shutdown is necessary to permit manual removal of material from plugged areas.
One prior art arrangement devised to overcome the problem of bridging in the lower tank is shown in U.S. Pat. No. 3,780,993 mentioned above. In this arrangement, the central mixing auger is provided with a lower flighting section having relatively wider outside dimensions. This configuration is designed to increase the volume of material displaced per revolution and thereby more effectively circulate material in the tank. Although this type of auger has been generally successful, the overall efficiency drops off under certain crop conditions due to shortcomings stemming from the conical configuration of the tank. This patent also disclose an auger having a plurality of elements extending outwardly from the edge of the lower flighting. As the central mixing auger is rotated, the extensions engage and agitate surrounding material to break up clusters of material in the vicinity of the auger and permit the material to gravitate more freely to the base of the tank. Consequently, the extensions further enhance overall circulation of feed material within the tank during a mixing operation. Although this feature generally improves mixing capability under adverse conditions, it is not entirely satisfactory in that it tends to create turbulence during circulation of dry crop materials peculiar to certain geographical regions.
Another prior art approach to the problem of bridging in the lower tank portion of grinder-mixers is disclosed in U.S. Pat. No. 3,667,734, to A. D. Skromme et al, issued June 2, 1972. In this patent, the tank walls of the grinder-mixer are vertical and when material gravitates to the bottom of the mixing chamber during operation, it is engaged by the blade of a long sweep arm rotating with and extending outwardly from the mixing auger shaft. The arm sweeps across a path parallel to and encompassing the entire bottom of the tank. A similar sweep arm is employed in the vertical mixing tank disclosed in U.S. Pat. No. 1,576,018, issued Mar. 9, 1926, to R. B. Wolf. In this type of apparatus the tendency of material to wedge under the sweep arm causes an upward force component with obvious deleterious effects. While this problem is partly alleviated by a guide used to restrain the outer end of the sweep arm, it is conceivable that this could compound the problem due to the tendency to bridge between the sidewall and any ledge-like extension from the wall into the tank. A further shortcoming of a vertical sidewall type configuration is the inherent need for a head of pressure, without which the arm has a tendency to merely slide under the material without changing its relative position in the tank, as is the situation during partial load conditions. This disadvantage becomes more critical at the end of an unloading operation when the decreased weight of the load reduces the sweep arm's ability to urge material toward an unloading port. Thus, the sweep arm seemingly operates best under full load conditions but conversely as the load increases the effects of wedging become more pronounced.
There exists still another prior art approach to the problem of bridging in grinder-mixers having an inverted conical shaped lower tank portion inclined inwardly for guiding material toward a centrally located mixing assembly. For example, apparatus is sold by the Koehring Farm Division of Des Moines, Iowa, wherein the mixing assembly comprises a cylindrical auger mounted for rotation with a lower auger portion having an inverse frusto-conical shaped core. In this arrangement the inclined wall commences at the approximate center of the tank and the bottom of the tank is completely covered by the lower core section as shown in Koehring brochure No. 15D472 NWC. A similar configuration is disclosed in U.S. Pat. No. 3,589,684, issued on June 29, 1971, to Bernard Dixon. The Dixon patent also shows a mixing tank with a vertical mixing assembly having a frusto-conical shaped lower core section on a common shaft with an upper cylindrical auger. The upper and lower augers are coaxially journalled in the tank for rotation within a housing that is flared at the bottom in a shape adapted to accommodate the conical portion of the lower auger. In Koehring the auger tube does not include a flared portion at the lower end thereof, and thereby exposes the conical portion of the auger to material being guided along the tank wall.
In the Dixon arrangement, even though the flared enclosure has slotted apertures, the conical portion of the housing would tend to cause bridging between it and the tank wall due to the convergence of gravitating material with no means to rapidly take it away. Bridging problems are exceptionally acute under conditions where crop material is damp as is the case in many areas of the world where grinder-mixers are employed. In the Koehring arrangement without a flared housing portion, even though material is allowed to contact the conical portion of the auger prior to reaching the bottom of the tank, the problem of bridging at the area of convergence between the vertical auger housing and the inner surface of the inclined sidewall is not embraced.